Tem mode coupler having an exponentially varying coefficient of coupling



3,390,356 YING June 25, 19

TEM

A. G. RYALS ET AL PLER HAVING AN EXPONENTIALLY JJ JJJ (-2 INVENTORSAUBER G. RYALS RICHARD W. ANDERSON BY a C ATTORNEY United States PatentOffice 3,390,356 Patented June 25, 1968 3,390,356 TEM MODE COUPLERHAVING AN EXPO- NENTIALLY VARYING COEFFICIENT OF COUPLING Auber G.Ryals, Palo Alto, and Richard W. Anderson,

Los Altos, Calif., assignors to Hewlett-Packard Company, Palo Alto,Calif., a corporation of California Filed July 30, 1965, Ser. No.475,955 12 Claims. (Cl. 333-9) ABSTRACT OF THE DISCLOSURE Primary andauxiliary inner conductors are mounted within an outer conductor intransverse electric and magnetic field coupling proximity. The spacingbetween the inner conductors increases at a constant angle in onedirection from a region of tight coupling to a region of looser couplingwhere the auxiliary conductor terminates in a load that is axiallyaligned therewith. This provides an exponentially varying coefficient ofcoupling between the inner conductors. An inner portion of the outerconductor protrudes between the inner conductors near the region oftight coupling at a point where the auxiliary conductor divergesabruptly from the primary conductor to a connector terminal mounted in aside of the outer conductor. This minimizes non-exponential couplingbetween the inner conductors. The cross-sectional area of the innerconductors and the spacing between the inner sidewalls of the outerconductor and the inner conductors vary to provide the inner conductorswith a substantially constant characteristic impedance and to providethe primary conductor With a low reflection transition into a pair ofconnector terminals mounted in the opposite ends of the outerconductor-s. An energy absorbent body is mounted within the outerconductor adjacent to at least one of the inner conductors to suppressspurious TE and TM modes that may be caused by the enlargement of theouter conductor to accommodate the increasing spacing of the innerconductors throughout the coupling region.

This invention relates to exponential TEM mode couplers.

It is the principal object of this invention to provide an exponentialTEM mode coupler having a substantially constant characteristicimpedance for operation over a very broad frequency range.

Another object of this invention is to provide a very broad-band TEMmode coupler in which spurious TE and TM modes are suppressed.

In accordance with the illustrated embodiment of this invention there isprovided an exponential TEM mode coupler comprising diverging primaryand auxiliary conductors positioned within an outer conductor intransverse electric and magnetic field coupling proximity to provide anexponentially varying coefiicient of coupling. The primary conductor isconnected between a pair of coaxial transmission line connectors mountedin the opposite end sections of the outer conductor. The spacing betweenthe inner surfaces of the opposite side-walls of the outer conductor isreduced at each end of the coupling region to provide the primaryconductor with a low reflection transition into the coaxial transmissionline connectors. A side-wall having a sloping inner surface ispositioned near the primary conductor, and the auxiliary cond-uctor isprovided with a flattened portion of varying cross-sectional area togive the TEM mode exponential coupler a substantially constantcharacteristic impedance. The auxiliary conductor terminates in a loadwhich is aligned therewith and mounted in the other side-wall of theouter conductor at the more loosely coupled end of the primary andauxiliary conductors. A body of absorbent material is mounted on theinner surface of this side-wall adjacent to the auxiliary conductor forsuppressing spurious TE and TM modes.

Other and incidental objects of this invention will become apparent froma reading of this specification and an inspection of the accompanyingfull scale drawing in which:

FIGURE 1 is a sectional top view of an exponential TEM mode coupler fora frequency range of from 0.5 to 12.4 kilomegacycles per second; and

FIGURE 2 is a sectional end view of the coupler of FIGURE 1 taken alongthe line AA.

Referring now to FIGURES 1 and 2, there is shown an exponential TEM modecoupler comprising circular primary and auxiliary conductors 10 and 12which are symmetrically supported between a pair of parallel groundplanes 14 to minimize the excitation of TE and TM modes. These primaryand auxiliary conductors 10 and 12 are positioned in transverse electricand magnetic field coupling relationship at an angle 0 over the length Lto provide an exponential variation of coupling coefficient K withdistance x measured from the tightly coupled end of the primary andauxiliary conductors 10 and 12 along the longitudinal axis of thecoupler. Thus,

where x is measured from the tightly coupled end of the primary andauxiliary conductors 10 and 12, K is the coupling coelficient at thistightly coupled end, and x is the distance along x in which K(x) isreduced by the factor F or, in other words, changes by 8.68 decibels.The angle 0 is generally made less than ten degrees to obtain gooddirectivity. As the angle 0 is decreased, the lower limit of theoperating range of the coupler is also de creased. The upper limit ofthe operating range of the coupler is theoretically infinite because ofthe assymmetrical exponential variations of K(x) with x, where couplingis initiated abruptly at the tightly-coupled end of conductors 10 and12. A smaller ripple of coupling coeflicient is obtained over theoperating range of the coupler for larger spacing between the looselycoupled ends of the primary and auxiliary conductors 10 and 12. Thus, itis normally desirable to have a small angle 0 and a sufficient length Lto provide a substantially greater spacing between the primary andauxiliary conductors 10 and 12 at their loosely-coupled end than attheir tightly-coupled end. This provides a very broad-band exponentialcoupler having a small ripple of coupling coefficient.

Primary conductor 10 is connected between and supported by a pair ofcoaxial transmission line connectors 16 mounted in the opposite endsections of side-Walls 18 and 20. The spacing between the inner surfacesof the side-walls 18 and 20 is greatly reduced at the opposite ends ofthe coupling region to provide the primary conductor 10 with a lowreflection rectangular transition 28 into the coaxial transmission lineconnectors 16. The loosely-coupled end of auxiliary conductor 12terminates in a load 22 which is aligned with the auxiliary conductor atthe angle 0 for good directivity and mounted in an end section ofside-wall 20. The tightly-coupled end of auxiliary conductor-s 12 turnsa corner 24 of radius r and is connected to a coaxial transmission lineconnector 16 mounted in side-wall 20. A tip portion 21 of side-wall 20protrudes between the primary and auxiliary conductors 10 and 12 nearthe point where the spacing between them begins to increase at theconstant angle 0. This tip portion 21 minimizes non-eponentia'l couplingbetween the primary and auxiliary conductors 10 and 12 and, as describedin Richard C. Harmons copending patent application Ser. No. 301,450entitled High Directivity TEM Mode Coupler, filed on Aug. 12, 1963, andissued as U.S. Patent 3,204,206 on Aug. 31, 1965, increases the 3directivity of the coupler. Side-walls 18 and 20 and the parallel groundplanes 14 form a rectangular outer conductor at ground potential. Theratio of the spacing w between the inner surfaces of side-walls 18 and20 to the spacing it between the inner surfaces of the parallel groundplanes 14 is large (generally between 2 and 10) thereby making thedimensions of the coupler less critical and making it easier toaccurately provide the desired characteristic impedance. However, thislarge ratio may allow excitation and propagation of TE and TM modes overthe broad operating frequency range of the exponential TEM mode coupler.Thus, a body of absorptive material 26 is positioned between the groundplanes 14 on the inner surface of side-wall 20 adjacent to the auxiliaryconductor 12 to damp these spurious TE and TM modes. The volume of thisbody of absorptive material 26 is selected to be sufiicient to damp allTE and TM modes over the broad operating frequency range of the couplerso as to eliminate their undesirable effect on the normal TEM modeoperation of the coupler. But a minimum spacing is maintained betweenthe outer surface of the body of absorptive material 26 and theauxiliary conductor 12 so that a negligible amount of energy is coupledfrom the normal TEM mode of operation. This minimum spacing normallyshould be at least equal to the spacing h between the parallel groundplanes 14. It should be noted that a body of absorptive material such asthat shown in the drawing might be used to suppress spurious TE and TMmodes in any broad-band TEM mode coupler in which they may appear.Side-wall 18 is positioned close enough to the primary conductor 10 sothat there is insufiicient room for the excitation and propagation of TEand TM modes between the primary conductor 10' and the side-wall 18.Thus, damping is only required on the auxiliary conductor side of thecoupler shown in the drawing.

The exponential variation of coupling coefiicient K wit-h distance xalong the longitudinal axis of the coupler has 'the effect of alteringthe characteristic impedance of the coupler. To compensate in part forthis effect and provide the primary conductor 10 with a substantiallyconstant characteristic impedance, side-wall 18 is provided with asloping inner surface so that the spacing between it and primaryconductor 10 increases with distance towards the tightly coupled end ofthe primary and auxiliary conductors 10 and 12. In the region oftightest coupling between the primary and auxiliary conductors '10 and12 a small portion 30 of the primary conductor 10 is tapered to providefurther compensation as required to maintain the desired characteristicimpedance along the primary conductor 10. This portion 30 of primaryconductor 10 is shown greatly exaggerated in FIGURE 1 of the scaledrawing, but it uniformly decreases in diameter by about four mils overone inch of length. In lieu of sloping the inner surface of side-wall18, the primary conductor 10 might be similarly tapered along the entirelength L thereof to provide a constant characteristics impedance alongthe primary conductor 10. The auxiliary conductor 12 is flattened on oneside in the region of tighest coupling to provide a portion 32 ofvarying cross-sectional area for maintaining a substantially constantcharacteristic impedance along the auxiliary conductor 12. This portionof the auxiliary conductor 12 is also shown greatly exaggerated inFIGURE 1 of the scale drawing, but it uniformly increases incross-sectional area on either side of the corner 24 where the outeredge of the auxiliary conductor 12 is machined away to a depth of aboutthirtyfive mils. It is much easier to flatten a portion of the auxiliaryconductor 12 on one side in this manner than to taper the auxiliaryconductor and provide a uniformly varying diameter.

We claim:

1. A broad-band TEM mode coupler comprising:

a first electromagnetic wave energy transmission path including a firstconductor positioned within an outer conductor; and

a second electromagnetic wave energy transmission path including asecond conductor positioned within said outer conductor;

said first and second conductors being positioned in transverse electricand magnetic field coupling proximity with a finite spacing between theadjacent edges of said conductors;

said finite spacing increasing with distance in one di rection alongsaid conductors to provide an exponentially varying coefiicient ofcoupling therebetween;

at least one of said first and second conductors having portions ofdifferent cross-sectional area to provide at least one of said first andsecond electromagnetic wave energy transmission paths with asubstantially constant characteristics impedance.

2. A broad-band TEM mode coupler as in claim 1 wherein:

said outer conductor has a sidewall positioned near said firstconductor; and

the spacing between said sidewall and said first conductor increaseswith distance in a reverse direction along said conductors to providesaid first electromagnetic wave energy transmission path with asubstantially constant characteristic impedance.

3. A broad-band TEM mode coupler as in claim 1 wherein:

a body of absorptive material is mounted on the inner surface of saidouter conductor adjacent to said second conductor for damping spuriousTE and TM modes in the operating range of said coupler;

said body of absorptive material being spaced from said second conductorto couple a minimal amount of energy from said TEM mode of operation.

4. A broad-band TEM mode coupler as in claim 1 wherein:

said first conductor is supported between and connected to a pair ofcoaxial transmission line connectors mounted in said outer conductor;and

the spacing between the sidewalls of said outer conductor is reducednear at least one end of the coupling region to provide a low reflectiontransition into at least one of said coaxial transmission lineconnectors for said first conductor.

5. A broad-band TEM mode coupler as in claim 1 including a load forterminating said second conductor near the region of loosest couplingbetween said first and second conductor, said load being axially alignedwith said second conductor.

6. A broad-band TEM mode coupler as in claim 1 including meanspositioned between said first and second conductors near the region oftightest coupling therebetween for reducing non-exponential couplingbetween said first and second conductors.

7. An exponential TEM mode coupler comprising:

a first electromognetic wave energy transmission path including a firstconductor mounted within an outer conductor; and

a second electromagnetic wave energy transmission path including asecond conductor mounted within said outer conductor;

said first and second conductors being positioned in transverse electricand magnetic field coupling proximity with a finite spacing between theadjacent edges of said conductors;

said finite spacing increasing with distance at a constant angle in onedirection along substantially the entire length of the coupling regionbetween said conductors to provide an exponentially varying coefiicientof coupling therebetween.

8. An exponential TEM mode coupler as in claim 7 including meanspositioned between said first and second conductors near the region oftightest coupling for reducing nonexponential coupling between saidfirst and second conductors.

9. An exponential TEM mode coupler as in claim 8 wherein said couplerincludes at least one energy absorbing body positioned within said outerconductor along a sidewall of the outer conductor and a finite distancefrom one of said first and second conductors to damp spurious TE and TMmodes.

10. An exponential TEM mode coupler as in claim 9 including a loadterminating said second conductor near the region of loosest coupling,said load being axially aligned with said second conductor at saidconstant angle with respect to said first conductor.

11. An exponential TEM mode coupler as in claim 10 wherein at least oneof said first and second conductors has portions of differentcr0ss-sectional area near the region of tightest coupling formaintaining a substantially constant characteristic impedance along atleast one of said electromagnetic wave energy transmission paths.

12. An exponential TEM mode coupler as in claim 11 wherein:

6 said first and second conductors are circular; and at least one ofsaid first and second conductors has a flattened portion near the regionof tightest coupling for maintaining a substantially constantcharacteristic impedance along at least one of said electromagnetic waveenergy transmission paths.

References Cited UNITED STATES PATENTS 2,794,959 6/1957 FOX 333-402,934,719 4/1960 Kyhl 333-10 3,012,210 12/1961 Nigg 333l0 3,166,7231/1965 Bock et a1. 333-10 H, K. SAALBACH, Primary Examiner.

PAUL L. GENSLER, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,390,356 a June 25, 1968 Auber G. Ryals et a1.

.It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2 line 25 "I((x) K e-xlx should read K(x) K e Column 2 line 36,"variations" should read variation line 66, "non-eponential" should readnon exponential Column 4, line 16, "characteristics" should readcharacteristic Signed and sealed this 11th day of November 1969.

S mean Edward M. Fletcher, Jr. R WILLIAM E. SCHUYLER, JR. LttestingOfficer Commissioner of Patents

